1. Field of the Invention
The present invention relates to improvements in a hydraulic control device for controlling a continuously variable transmission of a belt-and-pulley type for an automotive vehicle.
2. Discussion of the Prior Art
There is known a belt-and-pulley type continuously variable transmission for a motor vehicle which includes a pair of variable-diameter pulleys provided on a first and a second rotating shaft, respectively, a transmission belt which connects the pair of pulleys to transmit power from one of the first and second shafts to the other, a first and a second hydraulic actuator for changing effective diameters of the respective variable-diameter pulleys, and a shift control valve device for controlling flows of a working fluid into and from said hydraulic actuators, to change the speed ratio of the continuously variable transmission. An example of such a belt-and-pulley type continuously variable transmission is disclosed in laid-open Publication No. 52-98861 (published in 1977) of unexamined Japanese Patent Application.
An example of a hydraulic control device for controlling the speed ratio of the above-indicated type of continuously variable transmission uses a shift control valve device, which is electrically driven or controlled by a suitable electric controller to change the speed ratio of the transmission as needed. For instance, the electric controller applies a controlled drive signal to the shift control valve device such that the actual speed ratio of the transmission or the actual speed of its input shaft coincides with a determined target or desired value.
In the belt-and-pulley type continuously variable transmission, the range in which the speed ratio can be varied is determined, for example, by a maximum distance of movement of axially movable rotors of the variable-diameter pulleys, which in turn determins a maximum amount of change in the width of V-grooves of the pulleys which determines an effective diameter of the pulleys. In other words, the range of the speed ratio of the transmission is mechanically defined or determined. On the other hand, the controller may determine a target speed ratio which is outside the mechanically defined range. In this case, the shift control valve device is commanded to apply the line pressure to the hydraulic actuator on the drive or driven side, even after the actual speed ratio reaches the upper or lower limit of the mechanically defined range, in order to establish the determined target speed ratio, or so that the actual speed ratio coincides with the determined target value. In this case, the pressure in one of the two hydraulic actuators rises to a level higher than that required to establish the maximum or minimum speed ratio. Accordingly, a hydraulic power source in the form of an oil pump suffers from a power losss, and the transmission belt tends to be deteriorated due to an excessive tension. In the case where the hydraulic control device is adapted such that the line pressure is regulated by a pressure regulating valve based on the pressure in the hydraulic actuator, the pressure regulating valve raises the line pressure to a level higher than the pressure in the hydraulic actuator, when the ine pressure becomes equal to the pressure in the hydraulic actuator. That is, the line pressure may be raised to an unnecessarily high level. In this case, the problems such as the hydraulic power loss and deterioration of the belt as indicated above are aggravated.